DESCRIPTION (provided by investigator): Acute respiratory infections result in ~ 1.9 million childhood deaths per year, 20% of the deaths are in India. The disease burden of influenza among children in India is ill defined but is likely substantial. Influenza vaccines are in routine use in children in the US, although licensed in India, they are seldom given. Indeed, it is not known whether influenza vaccine efficacy in a tropical developing country will be the same as in developed countries. Specific Aim 1. Describe influenza vaccine direct and indirect efficacy. This aim will be accomplished by determining the efficacy of influenza vaccination in children six months of age through primary school on decreasing the incidence of laboratory-confirmed influenza in the children;and determining the indirect effects of influenza vaccination on laboratory-confirmed influenza among unvaccinated individuals who are living in the same communities in rural India using a prospective, group randomized, controlled, observer-blinded study. All individuals (children and adults) in both communities who participate in the study will be actively monitored by weekly visits for the occurrence of febrile acute respiratory illness (FARI, reported fever and any respiratory complaint). Each FARI will be further characterized on the basis of clinical signs and symptoms, including pulse oximetry, and a respiratory specimen will be collected. A major subset of FARIs will undergo realtime reverse transcriptase polymerase chain reaction testing for influenza, positive samples will be subjected to culture for virus isolation. Vaccine related adverse events will be monitored and reported. Specific Aim 2. Define influenza clinical disease and outcomes. This aim will be accomplished by defining the rates and epidemiological characteristics of influenza disease outcomes in persons of all ages who are living in the study communities, including the incidence of severe disease. The clinical presentations of influenza disease, including co-morbid conditions, influenza-related complications, and risk factors associated with more severe illness due to influenza will be identified. In addition to defining the direct and indirect protective effect of influenza vaccine, these studies will develop capacity for influenza virus study and management in second largest country in the world.